This invention relates to an electronically controlled exhaust gas recirculation system in an internal combustion engine.
In internal combustion engines and particularly in automotive engines, it is one of effective and prevailing methods for lessening nitrogen oxides (NOx) in the engine exhaust gas to recirculate a portion of the exhaust gas back to the engine combustion chambers. A suppresive effect of exhaust gas recirculation (EGR) augments as the rate of EGR is enhanced. The rate of EGR refers to the proportion of the quantity of the recirculated exhaust gas to the quantity of air taken into the engine or the proportion of the recirculated portion to the entire quantity of the engine exhaust gas. However, in general the performance of the engine represented by practically important items such as fuel economy, output characteristic and smoothness of operation is adversely affected by the recirculation of exhaust gas. Accordingly the rate of EGR should be controlled according to operating conditions of the engine so as to achieve satisfactory suppression of NOx emission without significantly impairing the performance of the engine.
Most of conventional EGR systems employ a pressure-responsive flow control valve as an EGR control valve and utilize a vacuum created in the engine intake system and/or exhaust pressure directly as a pressure signal for operating the control valve. This type of control valves have limitations on their precision due to tolerances permitted in the production, deposition of foreign matter thereon during use in exhaust gas and deterioration of certain components such as springs by long exposure to high temperatures of exhaust gas. A direct use of intake vacuum and/or exhaust pressure to operate the EGR control valve also puts limitations on both the accuracy and freedom of the control of EGR rate. For such reasons it has been difficult to optionally vary the rate of EGR so as to be always optimum under various operating conditions of the engine. Particularly, there is a tendency that the rate of EGR becomes excessively high under high speed low load conditions and low speed low-to-medium load conditions.